The invention relates to remote sensing systems, that is, systems having one or more sensors for sensing particular parameters or changes in such parameters and which are remote from a station which is intended to respond to information received from the sensor or sensors. One particular example of such a system is a security system in which there are a plurality of sensors situated at different positions in an area to be monitored (such as a building or house or part of a building such as a flat or apartment) and arranged to sense (for example) the presence of an intruder, the movement of an object, fire or smoke or inadvertent escape of a substance such as gas or water, each sensor being arranged to transmit signals representative of what it is sensing to a master station which is in or near the area being monitored and which then responds by taking appropriate action such as emitting a warning signal or causing such a signal to be transmitted to a distant location (as by means of a telephone line).
Many different forms of such security installations are known. It is common to instal such systems by connecting each sensor to the master station by wired links, the wires feeding power to the sensors and transmitting signals from them to the master station. Such arrangements have the advantage of providing secure and substantially interference-free transmission of signals. However, they are expensive to instal, mainly because of the problems involved in installing the wired links and, if these are not installed properly, they may be unacceptably unsightly.
It has been proposed to deal with this problem by using wireless links between sensors and the master station, so that signals are transmitted from the sensors to the master station by radio. If each sensor is, in addition, powered by its own battery, installation is very considerably simplified because, of course, there is no longer any need to instal wired links between each sensor and the master station. However, although the sensors are arranged to radiate their signals with low power (and this is normally required by Government regulations), it may be impossible (in an apartment block for example) to avoid signals radiated by the sensors in an installation in one apartment from reaching the master station of the installation in an adjacent apartment. It is therefore necessary to ensure that each master station only responds to signals from its own sensors.
However, the problem of signals radiated by the sensors in one installation reaching the master station of another installation can arise not only in systems transmitting signals by radio but also sometimes in systems transmitting signals in other ways, such as by means of infra-red radiation, ultrasonic vibrations or any other suitable means, including also by means of wired links and in particular in an AC line carrier system, that is where transmission is by the power supply mains.